Substantially, gear pumps consist of a housing with two intermeshing gears, which are arranged on shafts, at least one of the shafts being connected to a drive. The shafts are supported by slide bearings lubricated with pumping medium, which slide bearings are immediately arranged next to the internal space of the pump. The pumping medium used for the lubrication of the slide bearings gets from the pressure side via the gap of the slide bearing and a return duct into the suction side of the gear pump.
Gear pumps in particular, which are used for the conveying of low-viscous polymers and prepolymers and which comprise a dynamic sealing—in the form of a labyrinth sealing (sealing of threaded mandrel) for example—and subsequent static sealing—a packing sealing with or without sealing medium, for example—, must be ensured that always a positive pressure with respect to the suction side is present ahead of the dynamic mandrel sealing, since otherwise—in using a sealing medium—this can get into the pumping medium, which is highly undesirable. The positive pressure is necessary in order to get a sufficient filling of the sealing gap of the dynamic sealing. Thus, a penetration of sealing medium can be prevented into the main stream of the pumping medium.
On the other side, the pressure should not be too high in front of the dynamic mandrel sealing, since otherwise pumping medium can get outside via the dynamic mandrel sealing or—if a static sealing is present—the pumping medium gets in contact with this sealing, whereby a destruction of the static sealing must be expected.
Furthermore, it must be ensured that the return duct can be closed at the static sealing during maintenance work. For this reason, a valve has been provided in the return duct, by which a penetration of air into the suction side of the gear pump can be cut off.
However, the known valve is not suitable to meet the afore-mentioned conditions for the adjustment of the pressure of the pumping medium in front of the dynamic mandrel sealing. Thus, due to the adjustment characteristic of the known valve, it is uttermost difficult to adjust a pressure of a pumping medium in front of the dynamic sealing in complying with the afore-mentioned pressure conditions, since the range is very small, in which an adjustment must be made.
Therefore, the present invention has the object to provide a gear pump, which does not have the afore-mentioned drawbacks.
This object is solved by the present invention wherein the valve of the gear pump has an adjustment range, in which the pressure difference in function of the adjustment path has a slope between 0.05 and 2.5 bar per percentage of a maximum adjustment path, and wherein the adjustment range is at least 50% of the maximum adjustment path. Further embodiments of the present invention are described below.
The present invention relates to a gear pump consisting of a housing with at least two intermeshing gears, each with a shaft, which is supported by slide bearings lubricated with pumping medium. A pumping medium is conveyed from a suction side to a pressure side, and a return duct is provided, which leads pumping medium flowing to the outside via the slide bearing back to the suction side, and with a valve having a movable and a stationary part for the adjustment of a pressure difference in function of a adjustment path, which indicates a position between the stationary and the movable part. According to the present invention, the valve comprises an adjustment range, in which the pressure difference in function of the adjustment path comprises a slope between 0.05 and 2.5 bar per percentage of a maximum adjustment path. Furthermore, the adjustment range comprises at least 50% of the maximum adjustment path.
It arises as unit for the slope “bar per percentage of the maximum adjustment path xmax”. This unit is valid for all values indicated in this description for the slope for the course of the pressure difference in function of the adjustment path.
Therewith, a considerable improvement of the adjustment possibility of the pressure is obtained in the transition range between the slide bearing and a dynamic sealing of a drive shaft directed to the outside. In general, a good-natured adjustment characteristic has been obtained.
An embodiment of the gear pump according to the present invention is characterized in that the pressure difference in function of the adjustment path comprises a slope between 0.05 and 2 bar per percentage of the maximum adjustment path, particularly between 0.05 and 1.75 bar per percentage of the maximum adjustment path.
A further embodiment of the gear pump according to the present invention is characterized in that a closing range is provided, in which the pressure difference in function of the adjustment path is higher than 2.5 bar per percentage of the maximum adjustment path, the closing range comprising preferably 10 to 15% of the maximum adjustment path.
In a further embodiment of the present invention, the valve is contained in the return duct.
Alternatively, to the preceding embodiment of the present invention, the valve is contained in a feeding duct, which leads from the pressure side to the region arranged behind the slide bearing, viewed from the gears.
In an embodiment of the present invention, the valve comprises a pressure adjustment section, which mainly serves for the pressure adjustment. Furthermore, the valve comprises a closing section, by which the duct containing the valve can be opened or closed, respectively.
In a further embodiment of the present invention, the movable part is insert-able into the stationary part.
In another embodiment, the movable and the stationary part contact each other in the closing section if the duct containing the valve is closed.
In another embodiment of the present invention, the valve comprises a pressure adjustment section, which serves mainly for the adjustment of the pressure, and a closing section, in which the duct containing the valve can be opened or closed, respectively, the adjustment characteristic running linearly in the pressure adjustment section in a first approximation.
In a further embodiment of the present invention, the stationary part is an exchangeable sleeve.
In another further embodiment of the present invention, the valve comprises the following dimensions:                x: 0.5*D . . . 5*D, particularly 3*D;        S1: 0.008*D . . . 0.08*D;        di: di<D, di=D/1.5 . . . D/1.2;x being the adjustment path, D the diameter of the movable part, di the passage opening in the closing section, and S1 the gap width between the stationary and the movable part.        
In another embodiment of the present invention, the movable part is merely translatory displaceable.
In another embodiment of the present invention, a mandrel lifting drive is provided in order to displace the movable part in a translatory manner.
In another embodiment of the present invention, the movable part facing the end of the suction side is tapered, globular or flat.
A further embodiment of the present invention is characterized in that the movable part comprises one of the following cross-sections:                Polygon, particularly a triangle, quadrangle or hexagon;        oval;        round.        
Finally, a further embodiment of the present invention consists in that the closing section is provided after the pressure adjustment section in flow direction of the pumping medium.
The present invention is further explained with the aid of exemplified embodiments, which are shown in figures.